Oxygen is an electron acceptor. In the absence of oxygen (hypoxia) cells cannot generate ATP in the mitochondria. Instead, they will utilize glycolysis. Oxygen is required to carry out the electron transport chain and produce ATP via oxidative phosphorylation.

The electron transport system is the only metabolic process listed that directly requires molecular oxygen. Oxygen is the final electron acceptor (it is one of the most electronegative atoms in our bodies) in the electron transport chain. This is the same as saying that oxygen has the highest reduction potential, and is capable of receiving electons. If oxygen is not present to accept the electron from the final enzyme complex in the inner mitochondrial membrane, then electron transport will be inhibited and thus no ATP will be produced via chemiosmosis.

Note that the Krebs cycle, citric acid cycle, and tricarboxylic acid cycle (TCA cycle) all refer to the same process, and do not directly require oxygen (oxygen is neither a reactant nor a product in any of the steps). However, oxygen is indirectly required, as there is no point to this cycle without subsequent oxidative phosphorylation. Thus in the absence of oxygen, of the choices shown, only glycolysis will proceed uninhibited.

The electron transport chain generates the most ATP out of all three major phases of cellular respiration. Glycolysis produces a net of 2 ATP per molecule of glucose. In the Krebs cycle, there is one GTP (which is an ATP equivalent) generate in the conversion of succinyl-CoA to succinate. However, the majority of the ATP produced during cellular respiration occurs at the electron transport chain by the reduction of coenzymes NADH and . This subsequently results in the generation of the proton motive force which ATP synthase uses to generate ATP from one unit of ADP and one unit of inorganic phosphate.

In a eukaryote, oxidative phosphorylation occurs in the mitochondria because this is where the cell is able to set up a proton gradient.  However, prokaryotes do not have mitochondria - they have no membrane-bound organelles at all.  Therefore, the proton gradient that drives ATP synthesis in oxidative phosphorylation is created across the cell membrane.

With the excess buildup of protons in the matrix, the only thing that will be inhibited is the generation of ATP by ATP synthase. The other processes in cellular respiration focus more on creation of high energy electron carriers, and therefore will continue as normal.

Predictably, a gain in oxygen is known as oxidation, while a loss of oxygen is reduction.  Hydrogen follows the opposite pattern as oxidation: removing hydrogen is oxidation, while gaining hydrogen is reduction.  Therefore, the correct answer is that removing hydrogens is considered oxidation.

In order to differentiate between oxidation and reduction in terms of electron transfer, it is helpful to remember the phrase "LEO the tiger says GER".  A loss of electrons is oxidation, while a gain of electrons is reduction. 

During oxidative phosphorylation,  is created from the previously created  and . All of the other choices describe other parts of cellular respiration. In glycolysis, glucose is oxidized to pyruvate. In both glycolysis and the Krebs cycle, substrate level phosphorylation occurs. Likewise,  and  are produced during glycolysis and the Krebs cycle, but not during oxidative phosphorylation, where these high energy electrons are passed down a series of membrane-bound enzymes to oxygen meanwhile protons are pumped into the intermembrane space of the mitochondria.

To answer this question, it's important to have familiarity with the process of aerobic respiration.

In the first major pathway, glycolysis is split into two molecules of pyruvate through a series of reactions. Along the way, high-energy electron carriers are produced, along with ATP.

In the next major step, pyruvate is transferred into mitochondria, where it is decarboxylated into acetyl-CoA, with a concomitant production of NADH and carbon dioxide. Hence, this is a step that produces carbon dioxide. However, it is not found in the answer choices.

The third major component of aerobic respiration is the citric acid cycle. Here, the acetyl-CoA from the previous step is completely ripped apart to provide a great deal of energy. This huge amount of energy that is liberated is because the two carbon atoms that make up the acetyl group of acetyl-CoA become completely oxidized into two molecules of carbon dioxide. In terms of the energy liberated from the cycle, ATP along with a good deal of high-energy electron carriers are produced. This component of aerobic respiration is indeed a source of carbon dioxide.

Fermentation is an anaerobic pathway and is thus not the correct answer. Depending on the organism, carbon dioxide may or may not be produced.

Finally, aerobic respiration culminates in oxidative phosphorylation. Here, all of the high energy carriers from the previous steps are fed into the electron transport chain, resulting in the production of a great amount of ATP, the main energy currency of cells. In this final major step, it is oxygen gas that is produced, not carbon dioxide.

Glycolysis occurs in the cytoplasm. Pyruvate dehydrogenase complex occurs in the mitochondrial matrix. Krebs cycle occurs in the mitochondrial matrix. Electron transport chain protein complexes are embedded in the mitochondrial inner membrane.

Aconitase is the enzyme that catalyzes the conversion of citrate to isocitrate. This essential enzyme is vital in energy production, as it acts like an iron regulatory protein. The conversion of citrate to isocitrate is important since it is needed to react with isocitrate dehydrogenase.